Impact of Nutritional Iodine Optimization on Rates of Thyroid Hypoechogenicity and Autoimmune Thyroiditis: A Cross-Sectional,Comparative Study

Abstract

Background:

Since several countries have established mandatory food iodine fortification, there has been a decrease in rates of iodine deficiency disorders in parallel with an increase in prevalence of autoimmune thyroid diseases. This study compared the nutritional iodine status and the prevalence of autoimmune thyroiditis and thyroid hypoechogenicity on ultrasound in schoolchildren in São Paulo (Brazil) in two distinct periods of time in which fortified salt had different concentrations of iodine.

Methods:

We conducted a cross-sectional study evaluating 206 children aged 7–14 years and without a history of thyroid disease. Assessments included measurements of thyrotropin (TSH), free thyroxine, antithyroperoxidase (anti-TPO), and antithyroglobulin (anti-TG) antibodies, urinary iodine concentration, and thyroid ultrasound.

Results:

Mean urinary iodine concentration was 165.1 μg/L. Eleven children (5.3%) were diagnosed with autoimmune thyroiditis based on at least two of four criteria adopted in our study: positive anti-TPO or anti-TG antibody, hypoechogenicity of the thyroid parenchyma on ultrasound, and a TSH >4.0 μU/mL. Comparing our results with those from a similar study conducted during a period in which concentrations of iodine in the salt were higher (median urinary iodine concentration >300 μg/L), we observed a trend toward a lower prevalence of autoimmune thyroiditis, although no definitive conclusion could be established.

Conclusion:

The current nutritional iodine status in our cohort was within optimal levels and lower than levels found in 2003. The prevalence of autoimmune thyroiditis seems to be decreasing in parallel with a decrease in iodine intake, although we could not reach a definitive conclusion.

Introduction

An overall decrease in rates of iodine deficiency disorders (IDD) has been observed after several countries implemented mandatory food fortification with iodine. The number of countries with low iodine nutrition (reflected by urinary iodine concentration <100 μg/L) has decreased from 54 in 2003 to 32 in 2011. Some geographic areas with low iodine nutrition have even shifted to a status of excessive iodine intake (urinary iodine concentration >300 μg/L) (1). Although these changes have decreased the rates of thyroid diseases associated with low iodine nutrition such as goiter and hypothyroidism, they seem to be associated with a paradoxical increase in rates of other thyroid diseases, including chronic autoimmune thyroiditis (CAT) (2 –6).

Increases in iodine intake have been associated with the detection of antithyroid antibodies in genetically predisposed individuals, ranging from identification of low titers of antithyroglobulin antibodies (anti-TG) to elevated titers of anti-TG and antithyroperoxidase (anti-TPO) and, eventually, hypothyroidism. A recent study suggested that iodination of the thyroglobulin molecule facilitates the exposure of its most immunogenic epitope (7). Another possible mechanism involving iodine in the physiopathology of autoimmune thyroiditis includes changes in the production of free radicals by the thyroid. Chemotactic factors released during oxidative damage lead to recruitment of antigen-presenting cells, lymphocytic infiltration of the gland, and early injury to the thyrocyte (8,9). Despite evidence found in observational studies, the relationship between the increase in iodine ingestion and the increase in new cases of autoimmune thyroiditis is not yet proven, since there are no randomized clinical trials supporting this association.

Several studies have shown a correlation between cytological and histological findings with different patterns of ultrasound images in patients with autoimmune thyroiditis. Since the echogenicity of the parenchyma of the thyroid on ultrasound is determined by the amount of colloid within the follicles, the lower the amount of colloid resulting from antibody destruction of the follicles and the higher the degree of lymphocytic infiltration, the more hypoechoic (darker) the thyroid will appear on ultrasound (10 –13).

It is important to highlight that hypoechogenicity is often the only finding in the initial phases of autoimmune thyroiditis and may be present even before detection of serum antithyroid antibodies. In fact, thyroid hypoechogenicity has been shown to be more sensitive in predicting the development of hypothyroidism than the presence of antithyroid antibodies (14,15).

The prevalence of autoimmune disorders of the thyroid in different geographic areas varies due to genetic predisposition and nutritional iodine status of the population. In Spain, Garcia-Garcia et al. observed a prevalence of autoimmune thyroiditis in 3.7% of 1387 children between 1 and 16 years of age with optimal iodine nutrition (median urinary iodine concentration 199.5 μg/L) (16). In another cohort of 1000 children in Turkey, also with adequate iodine nutrition (median urinary iodine concentration 132 μg/L), the prevalence of thyroiditis (determined by the presence of goiter and antithyroid antibodies) was 3.6%. In this study, children with goiter and without thyroiditis presented a median iodine urinary concentration of 73 μg/L, whereas in the group of normal children, the concentration was 81 μg/L. This observation suggests an association between increased iodine ingestion and thyroiditis (17).

In Brazil, the prophylaxis of IDD is implemented primarily by iodized salt. Fortification of salt with iodine became mandatory in the country in 1956 and, since then, has undergone several adjustments. In 1999, the concentration of iodine in the salt increased from 40–60 mg to 40–100 mg per kg. Between 2002 and 2003, Duarte et al. conducted a study to obtain normative data of the thyroid volume in 964 children (boys and girls) between the ages of 6 and 14 years recruited from local schools in São Paulo, Brazil. In this study, the authors assessed the volume of the thyroid with ultrasound, and evaluated the nutritional iodine status of the cohort by measuring urinary iodine concentrations. Results from the study showed that 53% of these children had urinary iodine concentrations >300 μg/L (optimal level 100–299 μg/L). In 21% of them, the values were >600 μg/L. The authors also observed that 11.7% of the cohort had hypoechogenicity of the thyroid parenchyma on ultrasound. Considering that hypoechogenicity is an important marker of autoimmune thyroiditis, a possible association between iodine excess and autoimmune thyroiditis was considered in this cohort (18). Following that, two new adjustments of the iodine concentration in salt have been performed in Brazil. In 2003, the recommended iodine concentration decreased to 20–60 mg per kg of salt, and in 2013, it was decreased to 15–45 mg/kg (19). So far, the impact of these changes on the prevalence of autoimmune thyroiditis in the Brazilian population is unknown.

Based on this background, the aims of this cross-sectional study were to evaluate the nutritional iodine status of schoolchildren in São Paulo and to determine the rates of thyroid hypoechogenicity and autoimmune thyroiditis in the cohort. We also compared the results of the present study with those by Duarte et al. from 2003 (18), before the recommended concentration of iodine in salt decreased to 20–60 mg/kg. Our analysis may not only determine the impact of nutritional iodine adjustment on the thyroid, but also provide indirect evidence of the effect of the reduction on salt iodine on the prevalence of autoimmune thyroiditis in the overall Brazilian population.

Patients and Methods

Patients

The study included 206 schoolchildren (110 girls and 96 boys; ratio 1.14:1) between the ages of 7 and 14 years residing in the metropolitan area of São Paulo. Recruitment was performed between April 2012 and January 2013 during a screening interview for admittance to the emergency department (Pronto Socorro Infantil) at the Santa Casa de Misericórdia, a large community hospital in São Paulo, Brazil.

After a thorough history and physical examination of all children, we enrolled afebrile children in good general health presenting with minor acute health problems such as common cold, minor skin lesions, or conjunctivitis. These acute, mild manifestations are commonly found in children outside the hospital setting. This cohort, therefore, may be considered representative of the local population. Exclusion criteria for enrollment in the study included diagnosis of a genetic syndrome, type 1 diabetes, celiac disease, chronic diseases, known or suspected thyroid disease, or the use of medications and/or antiseptic preparations containing iodine within 30 days prior to enrollment.

We obtained written consent from legal guardians authorizing the participation of each child after offering detailed explanations about the objective and importance of the study. The study was approved by the Research Ethics Committee of the hospital.

All children underwent physical examination and had blood collected to determine thyroid function and levels of antithyroid antibodies. We measured urinary iodine concentrations in a spot urine sample from each child. We also performed thyroid ultrasound on all participants to analyze the echogenicity and estimate the volume of the gland.

Methods

Anthropometric measurements

We measured each child's weight in kilograms (kg) and height in centimeters (cm). Body surface area (BSA), expressed in m², was calculated with the formula:

BSA=weight (kg)^0.425×height (cm)^0.725×71.84×10^–4.

Body mass index (BMI), expressed in kg/m², was calculated with the formula:

BMI=weight (kg)/height² (m).

Thyroid function and antithyroid antibodies

Levels of thyrotropin (TSH), free thyroxine (fT4), and anti-TPO and anti-TG antibodies were determined by chemiluminescence (Immulite 2000, Siemens). Normal reference values are: TSH 0.4–4.0 μU/mL (sensitivity 0.03 μU/mL); fT4 0.9–1.8 ng/dL (sensitivity 0.25 ng/dL), anti-TPO antibody <35 IU/mL (sensitivity 5 IU/mL); anti-TG antibody <40 IU/mL (sensitivity 2.2 IU/mL).

Urinary iodine concentration

Urinary iodine concentrations were determined using the colorimetric reaction recommended by the International Council for Control of Iodine Deficiency Disorders (ICCIDD) (20), based on the reaction of Sandell-Kolthoff (21), modified by Pino et al. (22), and adopted by the laboratory Núcleo de Hematologia e Bioquímica do Centro de Patologia do Instituto Adolfo Lutz (23 –25). Values were determined after establishing an analytical curve using iodine calibrators with 20 μg/L, 50 μg/L, 100 μg/L, and 150 μg/L prepared with potassium iodate. For values>150 μg/L, we diluted the samples to assure linearity.

Thyroid ultrasound

Ultrasound (Esaote, Mylab 25 Gold) was performed with a 6–18 MHz linear transducer (LA 435). To determine the volume of the thyroid, we applied the ellipsoid formula:

longitudinal×transversal×anterior-posterior diameters (all in cm)×0.52 of each lobe and isthmus.

The total volume of each gland was calculated by the sum of the volumes of both lobes and isthmus and represented in milliliters (mL).

To determine the echogenicity of the thyroid, we used a visual analysis based on a gray scale that consisted in comparing the parenchyma of the gland with adjacent structures and classifying them into one of the two categories (26):

(1) Normal echogenicity: echogenicity similar or slightly hypoechoic when compared with the submandibular gland, but hyperechoic compared with cervical muscles.

(2) Hypoechogenicity: moderately or markedly hypoechoic when compared with the submandibular gland or with similar echogenicity of the cervical muscles. This category was considered one of the diagnostic criteria for CAT in our study (10,11,14,27).

To minimize variations in brightness, we adjusted the gain intensity of the device to values in which the echoes within the carotid artery and jugular vein were minimal. All ultrasound examinations were performed by the same physician (D.M.C.M.) to avoid inter-observer variation.

Criteria adopted for the diagnosis of CAT

Children with at least two of the four findings below were considered as having CAT (28): TSH above the upper normal limit (>4.0 μU/mL); positive anti-TPO antibody (>35 IU/mL); positive anti-TG antibody (>40 IU/mL); and moderate or marked hypoechogenicity of the thyroid parenchyma on ultrasound examination.

To estimate the nutritional iodine status of the cohort, we calculated the median urinary iodine concentration of all children. A median concentration level between 100 and 299 μg/L was considered indicative of optimal nutritional condition.

Statistical analysis

We performed descriptive and inferential analyses. In the descriptive analysis, we present absolute and relative frequencies for qualitative variables. For quantitative variables, we present summary measures: mean, median, standard deviation, and minimum and maximal values. To help visualization of the distribution of quantitative variables, we present charts and a boxplot diagram.

In order to analyze differences between the normal group and the group with autoimmune thyroiditis, we compared quantitative variables with analysis of variance (ANOVA) and qualitative variables with Fisher's exact test.

Point estimates and confidence intervals (CI) are presented to some parameters of interest.

We considered p-values of ≤0.05 as significant. All data were analyzed with SPSS for Windows v13.0 (SPSS, Inc.).

Results

Cohort description

As shown in Table 1, there were no differences between girls and boys in terms of age, height, weight, BMI, BSA, serum levels of TSH and fT4, thyroid volume, urinary iodine excretion, presence of anti-TPO and anti-TG antibodies, or rates of thyroid hypoechogenicity on ultrasound.

Table 1.

Cohort Description

						Comparison between boys and girls
	n	Mean	SD	Minimum	Maximum	p^*
Age (years)	206	10.2	1.9	7	14	0.451
Boys	96	10.3	2	7	14
Girls	110	10.1	1.8	7	14
Height (m)	206	1.42	0.12	1.14	1.7	0.398
Boys	96	1.42	0.13	1.17	1.7
Girls	110	1.4	0.11	1.14	1.66
Weight (kg)	206	39.2	12	19	82	0.624
Boys	96	39.64	11.4	19	82
Girls	110	38.82	12.5	19	76
BMI (kg/m²)	206	19.2	3.8	10.4	33.5	0.786
Boys	96	19.3	3.6	13.9	30.2
Girls	110	19.1	4	10.4	33.5
BSA (m²)	206	1.2	0.2	0.8	1.9	0.473
Boys	96	1.2	0.2	0.8	1.9
Girls	110	1.2	0.2	0.8	1.8
TSH (μU/mL)	206	2.2	1.6	0.2	13.5	0.326
Boys	96	2.1	1.5	0.2	11.5
Girls	110	2.3	1.7	0.38	13.5
fT4 (ng/dL)	206	1.1	0.2	0.9	1.5	0.786
Boys	96	1.1	0.1	0.9	1.5
Girls	110	1.1	0.2	0.9	1.5
Thyroid volume (mL)	206	5.3	1.9	1.8	13	0.906
Boys	96	5.3	2	1.8	10.9
Girls	110	5.2	1.9	2.3	13
Urinary iodine (μg/L)	202	165.1	20.6	100	200	0.231
Boys	95	166.9	19.7	107	199
Girls	107	163.4	21.3	100	200

				Between boys and girls
		Positive/total	%	p ^**
Anti-TG	Boys	1/96	1.0	0.375
	Girls	4/110	3.6
Anti-TPO	Boys	1/96	1.0	0.125
	Girls	6/110	5.5
Hypoechoic parenchyma	Boys	6/96	6.3	0.789
	Girls	9/110	8.2

^* Analysis of variance (ANOVA).

Fisher's exact test.

BMI, body mass index; BSA, body surface area; SD, standard deviation; TSH, thyrotropin; fT4, free thyroxine; anti-TG, antithyroglobulin antibodies; anti-TPO, antithyroperoxidase antibodies.

Iodine nutrition

Urine samples for the measurement of iodine concentration were available for 202 of the 206 children enrolled in the study.

Median and mean urinary iodine concentrations were 170.0 μg/L and 165.1 μg/L respectively (lowest 100.0 μg/L, highest 200.0 μg/L; Fig. 1).

FIG. 1.

Boxplot diagram showing the distribution of urinary iodine concentrations in 206 schoolchildren (7–14 years) in São Paulo, Brazil.

Thyroid function

Serum TSH levels were normal in 184 (89.3%) children, above the upper normal limit in 19 (9.2%), and below the normal limit in the remaining three children. All children had normal fT4 levels.

Thyroid ultrasound

Fifteen children had hypoechoic thyroid glands on ultrasound. The mean thyroid volume was 5.3 mL (SD=1.9 mL). When we compared thyroid volumes between boys and girls, the difference was not significant (p=0.906).

Comparison between results obtained in 2003 and 2012

Analyzing our results against those obtained by Duarte et al. with a comparable cohort of children (18), we observed a trend toward a decrease in the prevalence of hypoechogenicity (7.3% [CI 3.7–10.8] vs. 11.7% [CI 9.7–13.7] respectively) that paralleled a substantial decrease in urinary iodine concentration (165.1 μg/L vs. 459.6 μg/L respectively).

Prevalence of CAT

Based on two positive markers as diagnostic criteria in our study, we identified 11 children with CAT (5.3% [CI 2.3–8.4]; Table 2).

Table 2.

Distribution of Schoolchildren in São Paulo (Brazil) With and Without Chronic Autoimmune Thyroiditis According to the Frequency of Positive Markers

	N	%
Without CAT	195	94.7
Hypoechoic parenchyma with positive anti-TPO	2	1
Hypoechoic parenchyma with TSH >4.0 μU/mL	5	2.3
Hypoechoic parenchyma with positive anti-TPO and anti-TG	2	1
Hypoechoic parenchyma with positive anti-TPO, anti-TG, and TSH >4.0 μU/mL	2	1
Total	206	100

CAT, chronic autoimmune thyroiditis.

When we added children with only one positive marker to the analysis, we found a total of 19 children with a TSH >4.0 μU/mL, five with positive serum anti-TG antibodies, seven with positive anti-TPO antibodies, and 15 with thyroid hypoechogenicity on ultrasound (7.3% [CI 3.7–10.8]). Of note, children with a single positive marker were not considered as having CAT.

Eight girls and three boys received a diagnosis of CAT based on the criteria adopted in our study. There was no difference in the prevalence of CAT between boys and girls (p=0.132).

Table 3 shows the association of CAT with age, urinary iodine concentration, and thyroid volume. Of these, only increased thyroid volume correlated with presence of CAT (p=0.001), even after adjustment for BSA.

Table 3.

Association Between Chronic Autoimmune Thyroiditis, Age, Urinary Iodine Concentration and Thyroid Volume in Schoolchildren in São Paulo, Brazil

Variable		n	Mean	SD	Minimum	Maximum	p
Age (years)	Without CAT	195	10.2	1.9	7	14	0.996
n=206	CAT	11	10.2	1.6	7	13
Urinary iodine	Without CAT	191	164.9	20.2	100	200	0.598
(μg/L)	CAT	11	168.3	27.5	113	192
n=202
Volume (mL)	Without CAT	195	5.1	1.8	1.8	13.0	0.001
n=206	CAT	11	7.0	1.8	4.4	10.8

Discussion

After implementation of programs to prevent IDD, several countries including Brazil moved from a condition of chronic iodine deficiency to excessive iodine ingestion. This has contributed to increased rates of autoimmune thyroiditis, thyrotoxicosis, hypothyroidism, and goiter (1,29). Excessive dietary iodine may be a result of increased consumption of some types of food, such as Kombu algae in Japan (30), or certain foods with added iodine. In the Brazilian population, the main source of dietary iodine is iodized salt.

In 2003, Duarte et al. found a high average rate of urinary iodine concentration (459.6±204.0 μg/L) in schoolchildren during a period in which the concentration of iodine in salt was high, between 40 and 100 mg/kg (18). In contrast, schoolchildren in our study had urinary iodine concentrations within levels considered optimal (between 100 and 200 μg/L). It is reasonable to attribute this difference in urinary iodine concentration between both studies to a decrease in iodine concentration in salt to values between 20 and 60 mg/kg, which were implemented after collection of data by Duarte et al.(18), and reflecting a higher average consumption of iodine in the 2003 cohort compared with our cohort.

Following the decrease in urinary iodine concentration, the prevalence of CAT in our study (5.3% [CI 2.3–8.4]) was also lower than the one reported by Duarte et al. (11.7% [CI 9.7–13.7]), who adopted thyroid hypoechogenicity as a single criterion to estimate the rate of CAT. When we also considered hypoechogenicity alone as marker for CAT in our cohort, the estimate prevalence of CAT was 7.3% (CI 3.7–10.8). Although our lower mean estimate suggests a trend toward a decrease in hypoechogenicity, the confidence intervals in both estimates overlap, preventing us from establishing a definitive conclusion. It is worth mentioning that despite its reliability as a marker for autoimmune thyroid diseases, hypoechogenicity alone is unable to identify the type of disease involved, and may be present both in CAT as well as in Graves' disease (26).

All 11 children with CAT in our study had moderate to marked hypoechogenicity of the thyroid parenchyma on ultrasound. We also identified four additional children with similar degrees of hypoechogenicity. However, they were not included in the CAT group due to absence of other diagnostic criteria necessary to establish a diagnosis of CAT in our study. Still, it is very likely that these children also had autoimmune thyroid disease. Since changes in the parenchyma of the gland take place in the initial stages of the disease in patients with autoimmune thyroiditis, these changes can precede detection of circulating antibodies. This has been described by Rago et al. (14), who evaluated 458 volunteers with negative antithyroid antibodies, 29 of which had diffusely hypoechoic thyroid glands on ultrasound. Of these 29, four developed thyroid disease (two developed subclinical hypothyroidism and the others developed Graves' disease), and they had antibodies detected up to 36 months later. This was not observed in any of the 429 remaining individuals in whom the thyroid glands had normal echogenicity on initial evaluation (p<0.0001).

Two children in our study presented slight elevations in one of the antithyroid antibodies as the only positive marker. Therefore, they were not included in the CAT group. However, in this age group, even low titers of antibodies may reflect the presence of CAT. A similar interpretation can be extended to the 12 children who presented with TSH levels above the upper normal limit (28). These findings led us to think that some of these children may also have CAT and can potentially be diagnosed with the disease during follow-up.

It is important to stress that the different rates in CAT prevalence reported in areas of the world with adequate iodine nutrition are also influenced by genetic and ethnic factors, and the criteria adopted for the diagnosis of CAT. In Athens, the prevalence of CAT in a population of schoolchildren was 2.5%, in contrast to 9.6% in northwestern Greece (2). Both studies adopted as diagnostic criteria the association of positivity of at least one of the antithyroid antibodies with thyroid hypoechogenicity on ultrasound. When compared with Athens, the northwestern region of Greece became recently sufficient in iodine, raising the hypothesis that antithyroid antibodies may have increased temporarily in response to the increase in iodine consumption (2,31).

A similar finding was observed in Poland, where the frequency of new cases of thyroid autoimmune disease reduced from 30% to 10% five years after the population became iodine sufficient (32).

In Berlin, the prevalence of CAT was 3.4% (median urinary iodine concentration 139 μg of iodine per gram of creatinine), and 86% of the children with positive anti-TPO antibodies showed increased mean thyroid volumes (33).

In our study, the mean thyroid volume was greater in children with CAT when compared with children without CAT (p=0.001). This finding has also been reported in the cohorts studied in Berlin and Athens (31,33).

We did not find a significant difference in the average urinary iodine concentration between children diagnosed with CAT compared to those without markers of thyroid disease (p=0.598), which argues against the possibility that higher iodine nutrition was a potential trigger for autoimmune thyroiditis in these patients.

Considering our results, and the evidence that iodine ingestion seems now to be within optimal levels in our population, a few questions remain. What will be the impact of the new reduction in iodine concentration in table salt from 20–60 mg/kg to 15–45 mg/kg implemented in Brazil in April 2013? Will this reduction trigger again thyroid abnormalities caused by iodine deficiency? Will populations at risk, such as children and pregnant women, receive insufficient amounts of iodine? New studies will be necessary to determine the impact of the new mandatory iodine concentration in salt on the thyroid, particularly in individuals at higher risk for thyroid abnormalities associated with low consumption of iodine.

Conclusions

With a decrease in salt iodine concentration between 2003 and 2012 in Brazil, schoolchildren in São Paulo showed a parallel decrease in urinary iodine concentration to optimal levels.

Compared with the previous study in a similar population (18), our results show a trend toward a decreased prevalence of CAT, although no definitive conclusion can be established based on the current data.

Footnotes

Acknowledgments

We want to express our gratitude to Esaote do Brasil for providing the ultrasound equipment used in the study, and to Professor Ting Hui Ching, from the Faculdade de Ciências Médicas da Santa Casa de São Paulo, for her teaching and for providing the statistical analysis. Our sincere thanks also to the physician in charge of Pronto Socorro Infantil of Santa Casa de São Paulo, Silvio Luiz Zuquim and all the local staff, to the chief of the Pediatric Department of the institution, Rogério Pecchini, and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). Finally, we would like to thank Dr. Milena Braga-Basaria for the valuable comments and translation of the manuscript.

Author Disclosure Statement

The authors declare no conflicts of interest.

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